Radio receiving system



May 10, 1938. E. H. ARMSTRONG 1 2,116,502

RADIO RECEIVING SYSTEM Filed Aug. 25, 1956 2 Sheets$heet 2 R. m L. m m r i 2mg w r M k 8 Q a Q m m d wq 562 \QE 5:23 xifiw m f a 0 558 5533a MN Wm M mwm mma V 8 Q a MN kmst m i im t m E w M Patented May 10, 1938 UNITED STATES PATENT orrlce 6 Claims.

This invention relates to a method of improving the operation of the receiving apparatus described in my U. S. Patent No. 1,941,069. The

object of the invention is to improve the signal to' noise ratio so that communication may be effected over greater distances or with a minimum of power over a given distance. I

In the patent above referred to disturbances are eliminated by producing at the transmitter a frequency modulated wave having a frequency deviation which is greater than the frequency of the modulations to be transmitted and providing receiving means which are not responsive to amplitude variations and which respond substantiallyonly to frequency deviations of the magnitude of those produced by the transmitter.

As the frequency deviations which can be set up in the receiver by natural disturbances are relatively small at the modulation frequencies when compared to those produced by the transmitter, the response to natural disturbances is much reduced and the ratio of the amount of noise with respect to the signal is much improved. The operation of the system is-quantitative as long as the signal carrier is greater in amplitude than the amplitude of the disturbances by an amount which is determined by the characteristics of the limiter, and the signal to noise ratio can be reduced to practically any desired ratio by increasing the deviation of the transmitted wave with respect to the frequency of modulation and proportioning the design of the receiver to it. Under these conditions Where the signal carrier is greater in amplitude beyond a certain limit than the amplitude of the disturbing currents they usually manifest themselves in the speaker as a hiss of uniform amplitude. When, however, the amplitude of the disturbances becomes commensurate with the amplitude of the signal a form of interference occurs which cannot be so effectively dealt with by the receiver. This type of interference occurs when a disturbing current having substantially the amplitude of the signal arrives in opposite phase to the phase of the signaling current, the two cancel out and the current through the limiting system drops below the cutoff value. Where the disturbance is a repeated oscillation or has the nature of a spectrum a large number of such coincidences occur and they manifest themselves by producing a continuous rattle in the speaker.

There occurs also a type of interference different from either of the two mentioned which takes place when an impulse much larger than the signal arrives and blanks out the signal in the cuits which are of importance with respect to current limiter producing thereby a rapid change of the phase of the current passing through the current limiter which results in wide variations of frequency.

In my application for U. S. Patent Serial 5 10,544 there is described a method of improving the reception by the use of a local oscillator which is synchronized by means of, and kept in step with, the incoming signal so that the signaling -current ultimately applied to the limiter is in 10 this invention and Fig. 3 an alternative arrangement which may be used. Referring now to Fig. 1, I, 2 represents the antenna system, 3 a radio frequency amplifier, 4 a rectifier and 5 an oscil- 5 lator for heterodyning down to an intermediate frequency the incoming signal. 6 represents an aperiodic coil in the plate circuit of the rectifier 4, and l, 8 a tuned circuit coupled thereto with a resistance 9 for adjusting the damping, I0 is an amplifier coupled to the input of a band pass amplifyingsystem adjusted to cover the range over which it is desired to swing. A second rectifler l2 and oscillator l3 heterodyne the 1st intermediate frequency down to a 2nd intermediate value where it is amplified by the band pass amplifying system H, applied to the limiter I5, passed through the filter I6 for removing harmonies and supplied to the selective system H which converts the changes in frequency into changes in amplitude so that they may be rectified by the detector 18 to reproduce the original signal.

The general theory of operation may be understood from the following analysis: The tuned circuit 1, 8, 9 is adjusted to the mid frequency point so as to be in resonance with the signal frequency in the 1st intermediate stage when the carrier is unmodulated. The various disturbances which produce interference are most noticeable during periods of silence when there is no modulation on the carrier of the transmitted wave.

Under these conditions the carrier which is in resonance with the tuned circuit is raised in level v through the band passed by the band pass amplifiers. Hence at the current limiter the level of the carrier with respect to the disturbances is increased and the noise ratio improved. This improvement in the noise actually heard may be many times greater than the improvement in radio frequency voltage ratios as measured at the current limiter should the initial signal level be below the critical point heretofore referred to, as the effectiveness of the noise reduction in the current limiter itself increases rapidly with improvement in the signal to noise voltage ratios. When the carrier is modulated the advantage just described is reduced but under such conditions the noise is not so noticeable as the sound of the speech or music is usually much louder than the disturbance. It should be observed that the resonance of the tuned circuit should not be made too great as there will then exist therein transient oscillations which introduce distortions in the reproduction. By properly adjusting the constants of the circuit I, 8, 9 the most effective characteristics of the circuit may be obtained.

Fig. 2 illustrates the general relation between the characteristics of the tuned circuit and the band pass amplifier. It should be observed that the characteristics of the tuned circuit off res.- onance must be such that it does not reduce the level of the signaling current to a point below that at which the current limiter becomes inoperative.

Fig. 3 shows the same arrangement as Fig. 1 except that in this case the tuned circuit is placed at the beginning of the 2nd intermediate frequency amplifier as illustrated by 2Q, 30, 3|. The same phenomena take place in this arrangement and the relative shapes of the characteristics shown in Fig. 2 apply equally well to this case.

I have described what I believe to bethe best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

l. The method of reducing'interference in the radio reception of a wide-band frequency modulated wave, which consists in receiving'the wave, re-enforcing selected frequencies of the received wave with respect to the other frequencies lying within the admittance band of the receiver, removing the effects of amplitude modulations and detecting the frequency changes of the received wave.

2. Apparatus for reducing interference in the radio reception of a wide band frequency-modulated wave, comprising means for reinforcing selected frequencies of the received wave with respect to the other frequencies lying with the admittance band of the receiver, means for removing the effects of amplitude modulations and means for detecting the frequency changes of the received wave.

3. Apparatus for reducing interference in the radio reception of a wide band frequency modulated wave, comprising a receiver having a plurality of intermediate frequency stages, means in one of said intermediate frequency stages for reinforcing selected frequencies of the received wave with respect to the other frequencies lying within the admittance band of the receiver, means for removing the effects of amplitude modulations from the output of said second intermediate frequency stage, and means for thereafter detecting the frequency changes of the received wave.

4. The apparatus of claim 3 in which said means for reinforcing selected frequencies of the received wave is located in the first of said intermediate frequency stages.

5. The apparatus of claim 3 in-which said means for reinforcing selected frequencies of the received wave is located in the second of said intermediate frequency stages.

6. Apparatus for reducing interference in th radio reception of a wide band frequency-modulated wave, comprising a selective circuit tuned to approximately the'mid-frequency of said wide band for reinforcing selected frequencies of the received wave with respect to the other frequencies lying within the admittance band of the receiver, means for removing the effects of amplitude modulations and means for detecting the frequency changes of the received wave.

EDWIN H. ARMSTRONG. 

